Generating broadband quadrature-modulated signals presents a number of challenges in achieving wide bandwidth and spectral purity, in shaping the waveform, in eliminating spurious components and non-linearities from the delivered signal, and in calibrating the quadrature modulation. Receiving and demodulating the signals presents similar challenges, particularly in cases of frequency conversion relating to image rejection and local oscillator (LO) leakage at the intermediate frequency (IF). Another issue involves quadrature imbalance at the local oscillator, where imbalances occurring at different points in the transmit/receive path are typically inseparable and are therefore not readily correctible. Thus, it would be desirable to have apparatus and methods for generating and receiving quadrature-modulated signals having not only wide bandwidth and spectral purity, but also featuring ease of calibration and rejection of undesirable artifacts in the signal. This goal is met by embodiments of the present invention.